This invention relates to millimeter wave modules (MMW), and more particularly, this invention relates to a millimeter wave module for microwave monolithic integrated circuits (MMIC).
Millimeter wave (MMW) modules are becoming more commonplace as increasing use is made of millimeter wave transceivers and similar millimeter wave devices. Often, these modules are used with various transceiver designs having different transmitter and receive circuits that make use of a number of different microwave monolithic integrated circuit (MMIC) chips or die. Many of the MMIC chips are formed from Galium Arsenide (GaAs). These chips are often attached on an alumina, i.e., aluminum oxide, or similar dielectric substrate. Because of the extreme tolerances and necessity for thermal matching, millimeter wave modules typically use an expensive coefficient of thermal expansion (CTE) matched housing material, such as copper tungsten (CuW) or aluminum: silicon carbide (AlSiC) to mount Galium Arsenide MMIC chips and the alumina or similar dielectric substrates. CTE matching is required to prevent the MMIC chips and the substrates on which they are attached from cracking as the housing material shrinks and expands during extreme temperature variations.
Most millimeter wave Galium Arsenide MMIC dies or chips and the accompanying substrates have a Coefficient of Thermal Expansion (CTE) that is between about 4 and 6 ppm/deg. Centigrade. This has required the use of similarly matched housing materials, such as the copper tungsten or aluminum silicon carbide materials. Unfortunately, the lower cost housings, such as formed from aluminum and other similar metallic or other materials, have a very high coefficient of thermal expansion, greater than about 20 ppm/deg. Centigrade. As greater uses are made for millimeter wave modules, such as millimeter wave transceivers, it would be advantageous if a unique structure and method could be found that interfaced an inexpensive aluminum or other housing and the substrate and MMIC chip material, both having different coefficients of thermal expansions, without damaging or impacting the performance of any MMIC or other material used in the module and/or transceiver.
The present invention advantageously provides a unique structure and method to interface a housing and substrate material, both having a different coefficient of thermal expansion, without damaging or impacting the performance of the MMIC chip, RF interconnects and other material components. In accordance with one aspect of the present invention, a millimeter wave (MMW) module for a microwave monolithic integrated circuit (MMIC) includes a carrier board formed of a dielectric material and having at least one MMIC die (chip) mounted thereon, and at least one interface line. A base plate is formed of a material that has a higher, unmatched coefficient thermal expansion (CTE) than the carrier board. The base plate supports the carrier board.
A housing is mounted over the carrier board and engages the base plate. This housing has at least one waveguide or subminiature coaxial connector interface, also commonly referred to as an SMA connector, mounted thereon. A flexible circuit interconnect connects the subminiature coaxial connector(s) and the MMIC die through the interface line. A thermal interface member is positioned between the carrier board and base plate to aid in heat transfer between the base plate and housing and the lower CTE carrier board.
In still another aspect of the present invention, the flexible circuit interconnect comprises one of fuzz buttons or spring loaded self-adjusting interconnects, including the use of modified forms of pogo pins and similar spring segments and resilient members. The carrier board preferably, but not necessarily, comprises a plurality of layers of low temperature transfer tape (LTTT) to form a multilayer substrate board. The base plate and housing are formed from a material such as aluminum and/or similar metallic material. The thermal interface member comprises a heat transfer gasket that is formed from one of at least a phase change material, thermally conductive elastomer, or thermally conductive paste. Fasteners can secure the base plate and housing together.
In yet another aspect of the present invention, at least one alignment member is mounted on the base plate. The carrier board has a guide receiver that receives the alignment member for aligning the carrier board relative to the base plate without damage due to CTE mismatch. At least one alignment member includes an alignment pin. The housing is preferably formed of a material having a coefficient of thermal expansion substantially matching the base plate. The interface line comprises at least one 50 Ohm microstrip interface line as is commonly used and compatible with the 50 Ohm SMA interfaces and fuzz buttons.